Acoustic window with liquid-filled pores for chemical mechanical polishing and methods of forming pads

ABSTRACT

A chemical mechanical polishing apparatus includes a platen, a polishing pad supported on the platen, a carrier head to hold a surface of a substrate against a polishing surface of the polishing pad, and a motor to generate relative motion between the platen and the carrier head so as to polish an overlying layer of the substrate. The polishing pad includes a polishing layer of a solid matrix material with liquid-filled pores, and a backing layer. An in-situ acoustic monitoring system includes an acoustic sensor coupled to the backing layer to receive acoustic signals from the substrate, and a controller is configured to detect a polishing transition point based on received acoustic signals from the in-situ acoustic monitoring system.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority to U.S. Application No.63/218,897, filed on Jul. 6, 2021, the contents of which are herebyincorporated by reference.

TECHNICAL FIELD

This disclosure relates to in-situ monitoring of chemical mechanicalpolishing, and more particularly to in-situ acoustic monitoring duringpolishing.

BACKGROUND

An integrated circuit is typically formed on a substrate by thesequential deposition of conductive, semiconductive, or insulativelayers on a silicon wafer. One fabrication step involves depositing afiller layer over a non-planar surface and planarizing the filler layer.For certain applications, the filler layer is planarized until the topsurface of a patterned layer is exposed. A conductive filler layer, forexample, can be deposited on a patterned insulative layer to fill thetrenches or holes in the insulative layer. After planarization, theportions of the metallic layer remaining between the raised pattern ofthe insulative layer form vias, plugs, and lines that provide conductivepaths between thin film circuits on the substrate. For otherapplications, such as oxide polishing, the filler layer is planarizedtime period, e.g., by polishing for a predetermined time period, toleave a portion of the filler layer over the nonplanar surface. Inaddition, planarization of the substrate surface is usually required forphotolithography.

Chemical mechanical polishing (CMP) is one accepted method ofplanarization. This planarization method typically requires that thesubstrate be mounted on a carrier or polishing head. The exposed surfaceof the substrate is typically placed against a rotating polishing pad.The carrier head provides a controllable load on the substrate to pushit against the polishing pad. An abrasive polishing slurry is typicallysupplied to the surface of the polishing pad.

One problem in CMP is determining whether the polishing process iscomplete, such as when a polishing endpoint has been reached, e.g.,whether a substrate layer has been planarized to a desired flatness orthickness, when a desired amount of material has been removed, or whenan underlying layer has been exposed. Variations in the slurrydistribution, the polishing pad condition, the relative speed betweenthe polishing pad and the substrate, and the load on the substrate cancause variations in the material removal rate. These variations, as wellas variations in the initial thickness of the substrate layer, causevariations in the time needed to reach the polishing endpoint.Therefore, the polishing endpoint usually cannot be determined merely asa function of polishing time.

In some systems, the substrate is monitored in-situ during polishing,e.g., by monitoring the torque required by a motor to rotate the platenor carrier head. Acoustic monitoring of polishing has also beenproposed.

SUMMARY

In one aspect, a polishing pad includes a polishing layer, a backinglayer, and an acoustic window of solid material having an acousticimpedance less than that of the backing layer and extending through thebacking layer to contact the bottom surface of the polishing layer.

In another aspect, a polishing pad includes a polishing layer, a backinglayer, and an acoustic window extending through the backing layer andthe polishing layer. An upper surface of the acoustic window is coplanarwith a polishing surface of the polishing layer.

In another aspect, a chemical mechanical polishing apparatus includes aplaten, a polishing pad of either of the two aspects above supported onthe platen, a carrier head to hold a substrate against the polishingpad, a motor to generate relative motion between the platen and thecarrier head so as to polish an overlying layer of the substrate, anin-situ acoustic monitoring system including an acoustic sensor coupledto the acoustic window to receive acoustic signals from the substrate,and a controller configured to detect a polishing transition point basedon received acoustic signals from the in-situ acoustic monitoringsystem.

In another aspect, a chemical mechanical polishing apparatus includes aplaten, a polishing pad supported on the platen, a carrier head to holda surface of a substrate against a polishing surface of the polishingpad, and a motor to generate relative motion between the platen and thecarrier head so as to polish an overlying layer of the substrate. Thepolishing pad includes a polishing layer including a solid matrixmaterial with liquid-filled pores, and a backing layer. An in-situacoustic monitoring system includes an acoustic sensor coupled to thebacking layer to receive acoustic signals from the substrate, and acontroller is configured to detect a polishing transition point based onreceived acoustic signals from the in-situ acoustic monitoring system.

In another aspect, a method of fabricating a polishing pad includessuccessively depositing a first plurality of layers with a 3D printer toform a backing layer of the polishing pad, and successively depositing asecond plurality of layers with the 3D printer to form a polishing layerof the polishing pad on the backing layer. Each layer of the firstplurality of layers including a backing material portion and an acousticwindow portion having a lower acoustic impedance than the backingmaterial portion. The backing material portion and the acoustic windowportion are deposited by ejecting a plurality of precursor materialsfrom one or more nozzles and solidifying the plurality of precursormaterials to form a solidified backing material and a solidifiedacoustic window. Ejecting the plurality of precursor materials from theone or more nozzles forms an interface with intermingled polymer thatdirectly bonds the solidified acoustic window and the solidified backingmaterial.

Implementations may include one or more of the following features. Thewindow portion need not include the first precursor material. Thebacking material portion need not include the second precursor material.Successively depositing the second plurality of layers with the 3Dprinter to form the polishing layer may include ejecting a thirdprecursor not present in the backing material portion and/or the windowportion.

In another aspect, method for manufacturing a polishing pad includesdepositing a backing layer of a first material, curing the backinglayer, depositing a polishing layer of a second material atop thebacking layer, curing the polishing layer, removing a portion of thebacking layer creating an aperture, and inserting an acoustic window ofa third material into the aperture.

Implementations may include one or more of the following features. Thefirst material may be non-porous. The third material may be non-porous.

One or more of the following possible advantages may be realized. Signalstrength of an acoustic sensor can be increased. Exposure of anunderlying layer can be detected more reliably. Polishing can be haltedmore reliably, and wafer-to-wafer uniformity can be improved. Thestability of the signal intensity can be improved, both over short timescales, e.g,—due to greater control over the mechanical properties ofthe material above the sensor, and over longer time scales, e.g., due togreater control over transmission temperature dependence.

The acoustic window in contact with the transducer can be sized to matchthe transducer to increase sensitivity. Alternatively, the acousticwindow can have a reduced diameter to reduce the effective “spot size”of the transducer and data collected therefrom.

The details of one or more implementations are set forth in theaccompanying drawings and the description below. Other aspects,features, and advantages will be apparent from the description anddrawings, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a schematic cross-sectional view of an example of apolishing apparatus.

FIG. 2A illustrates a schematic cross-sectional view of a polishing padhaving an acoustic window in contact with an acoustic sensor.

FIG. 2B illustrates a schematic cross-sectional view of anotherimplementation of a polishing pad having an acoustic window in contactwith an acoustic sensor.

FIG. 2C illustrates a schematic cross-sectional view of animplementation of a polishing pad having an acoustic window extendingthrough the polishing and backing layers of the polishing pad.

FIG. 2D illustrates a schematic cross-sectional view of anotherimplementation of a polishing pad having an acoustic window extendingthrough the polishing and backing layers of the polishing pad.

FIG. 2E illustrates a schematic cross-sectional view of anotherimplementation of a polishing pad in contact with an acoustic sensor.

FIG. 2F illustrates a schematic cross-sectional view of an acousticwindow having liquid-filled pores.

FIG. 3 illustrates a schematic top view of a polishing pad havingmultiple acoustic windows.

FIG. 4 illustrates a schematic top view of a polishing pad having asolid portion surrounding an acoustic window.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

In some semiconductor chip fabrication processes an overlying layer,e.g., metal, silicon oxide or polysilicon, is polished until anunderlying layer, e.g., a dielectric, such as silicon oxide, siliconnitride or a high-K dielectric, is exposed. For some applications, whenthe underlying layer is exposed, the acoustic emissions from thesubstrate will change. The polishing endpoint can be determined bydetecting this change in acoustic signal. However, existing monitoringtechniques may not satisfy increasing demands of semiconductor devicemanufacturers.

The acoustic emissions to be monitored can be caused by energy releasedwhen the substrate material undergoes deformation, and the resultingacoustic spectrum is related to the material properties of thesubstrate. Without being limited to any particular theory, possiblesources of this energy, also termed “stress energy”, and itscharacteristic frequencies include breakage of chemical bonds,characteristic phonon frequencies, slip-stick mechanisms, etc. It may benoted that this stress energy acoustic effect is not the same as noisegenerated by vibrations induced by friction of the substrate against thepolishing pad (which is also sometimes referred to as an acousticsignal), or of noise generated by cracking, chipping, breakage orsimilar generation of defects on the substrate. The stress energy can bedistinguished from other acoustic signals, e.g., from friction of thesubstrate against the polishing pad or of noise generated by generationof defects on the substrate, through appropriate filtering. For example,the signal from the acoustic sensor can be compared to a signal measuredfrom a test substrate that is known to represent stress energy.

However, a potential problem with acoustic monitoring is transmission ofthe acoustic signal to the sensor. Acoustic emissions caused by stressenergy can be subject to significant noise, so a strong signal isneeded. However, conventional polishing pads, e.g., with porouspolishing and backing layers, tend to dampen the signal.

Thus, it would be advantageous to utilize a polishing pad with lowattenuation of the acoustic signal to decrease noise in the acousticsignal. A polishing pad including an acoustic window having beneficialacoustic properties such as low acoustic attenuation facilitatesacoustic signal transmission to the acoustic sensor, reducing signalnoise. Additionally, the acoustic window having compressive propertiessimilar to the surrounding polishing pad layer, e.g., the polishinglayer, or the backing layer, reduce acoustic signal reflection due toadjacent boundaries and maintains polishing characteristics of thepolishing pad. Any of these advantages could be used independent of theother advantages.

FIG. 1 illustrates an example of a polishing apparatus 100. Thepolishing apparatus 100 includes a rotatable disk-shaped platen 120 onwhich a polishing pad 110 is situated. The polishing pad 110 can be atwo-layer polishing pad with an outer polishing layer 112 and a softerbacking layer 114. The platen is operable to rotate about an axis 125.For example, a motor 121, e.g., a DC induction motor, can turn a driveshaft 124 to rotate the platen 120.

The polishing apparatus 100 can include a port 130 to dispense polishingliquid 132, such as abrasive slurry, onto the polishing pad 110 to thepad. The polishing apparatus can also include a polishing padconditioner to abrade the polishing pad 110 to maintain the polishingpad 110 in a consistent abrasive state.

The polishing apparatus 100 includes at least one carrier head 140. Thecarrier head 140 is operable to hold a substrate 10 against thepolishing pad 110. Each carrier head 140 can have independent control ofthe polishing parameters, for example pressure, associated with eachrespective substrate.

The carrier head 140 can include a retaining ring 142 to retain thesubstrate 10 below a flexible membrane 144. The carrier head 140 alsoincludes one or more independently controllable pressurizable chambersdefined by the membrane, e.g., three chambers 146 a-146 c, which canapply independently controllable pressurizes to associated zones on theflexible membrane 144 and thus on the substrate 10 (see FIG. 1 ).Although only three chambers are illustrated in FIG. 1 for ease ofillustration, there could be one or two chambers, or four or morechambers, e.g., five chambers.

The carrier head 140 is suspended from a support structure 150, e.g., acarousel or track, and is connected by a drive shaft 152 to a carrierhead rotation motor 154, e.g., a DC induction motor, so that the carrierhead can rotate about an axis 155. Optionally each carrier head 140 canoscillate laterally, e.g., on sliders on the carousel 150, or byrotational oscillation of the carousel itself, or by sliding along thetrack. In typical operation, the platen is rotated about its centralaxis 125, and each carrier head is rotated about its central axis 155and translated laterally across the top surface of the polishing pad.

A controller 190, such as a programmable computer, is connected to themotors 121, 154 to control the rotation rate of the platen 120 andcarrier head 140. For example, each motor can include an encoder thatmeasures the rotation rate of the associated drive shaft. A feedbackcontrol circuit, which could be in the motor itself, part of thecontroller, or a separate circuit, receives the measured rotation ratefrom the encoder and adjusts the current supplied to the motor to ensurethat the rotation rate of the drive shaft matches at a rotation ratereceived from the controller.

The polishing apparatus 100 includes at least one in-situ acousticmonitoring system 160. The in-situ acoustic monitoring system 160includes one or more acoustic signal sensors 162 and, in someimplementations, one or more acoustic signal generators 163 that areeach configured to actively transmit acoustic energy toward a side ofthe substrate 10 closer to the polishing pad 110. Each acoustic signalsensor or acoustic signal generator can be installed at one or morelocations on the upper platen 120. In particular, the in-situ acousticmonitoring system can be configured to detect acoustic emissions causedby stress energy when the material of the substrate 10 undergoesdeformation and, in implementations where acoustic signal generators 163are included, to detect the reflection of actively generated acousticsignals from the surface of the substrate 10.

A position sensor, e.g., an optical interrupter connected to the rim ofthe platen or a rotary encoder, can be used to sense the angularposition of the platen 120. This permits only portions of the signalmeasured when the sensor 162 is in proximity to the substrate, e.g.,when the sensor 162 is below the carrier head or substrate, to be usedin endpoint detection.

In the implementation shown in FIG. 1 , the acoustic sensor 162 ispositioned in a recess 164 in the platen 120 and is positioned toreceive acoustic signals through an acoustic window 118. The acousticsensor 162 can be connected by circuitry 168 to a power supply and/orother signal processing electronics 166 through a rotary coupling, e.g.,a mercury slip ring. The signal processing electronics 166 can beconnected in turn to the controller 190, which can be additionallyconfigured to control the magnitude or frequency of the acoustic energytransmitted by the generator 163, e.g., by variably increasing ordecreasing the current supply to the generator 163.

The in-situ acoustic monitoring system 160 can be a passive acousticmonitoring system. The passive acoustic signals monitored by theacoustic sensor 162 can be in 50 kHz to 1 MHz range, e.g., 200 to 400kHz, or 200 Khz to 1 MHz. For example, for monitoring of polishing ofinter-layer dielectric (ILD) in a shallow trench isolation (STI), afrequency range of 225 kHz to 350 kHz can be monitored. As anotherexample, passive mode frequencies of interest range from 500 kHz to 900kHz.

Referring to FIGS. 2A-2E, the polishing pad 100 includes a polishinglayer 112 and a backing layer (sometimes referred to as a subpad) 114.The backing layer 114 is more compressible than the polishing layer 112.

In some implementations, a plurality of slurry-transport grooves 116 areformed in the polishing surface 112 a of the polishing layer 112 of thepolishing pad 110. The grooves 116 can extend partially but not entirelythrough the thickness of the polishing layer 112. Alternatively, thegrooves 116 can extend entirely through the polishing layer 112. Forexample, the polishing layer 112 can be formed as a plurality ofdiscrete segments that sit on the backing layer 114. In someimplementations, the discrete segments of the polishing layer 112 extendinto recesses in the backing layer 114.

In some implementations, grooving is omitted from the polishing surface112 a in a region 119 that is aligned with the acoustic sensor 162. Theregion 119 lacking grooves can be wider than the pitch between thegrooves in the remaining region of the polishing layer. At least onegroove can be interrupted, e.g., at least one groove in an otherwiserectangular array of grooves does not extend entirely across thepolishing surface, or at least one groove in an otherwise concentriccircular array of grooves does not extend entirely around the centralaxis. However, for any of the implementations discussed below it ispossible to not omit the grooving from the region above the acousticsensor 162.

In any of the implementations of FIGS. 2A-2E, the polishing layer 112can be composed of a solid polymer matrix material 200 withliquid-filled pores 202. The matrix material 200 can be a polyurethaneor mixture of polyurethanes, and the pores 202 can filled with waterand/or a gel, e.g., a polymer gel. However, in some implementations thepores 202 in the polishing layer 112 are air-filled, e.g., provided byhollow microspheres. In some implementations, the pores 202 in theregion 119 aligned with the acoustic sensor 162 are liquid-filled, butthe pores 202 in other portions of the polishing layer 112 areair-filled.

In some implementations, the matrix material 200 of the polishing layer112 and the gel in the pores 202 include different components, e.g.,different polymers, that provide the different phases, i.e., liquidversus solid. However, in some implementations the matrix material 200and the gel in the pores 202 are formed using the same two monomer orpolymer components, but in different weight percentage contributions soas to provide the different phases. For example, assuming that a firstmonomer component polymerizes and solidifies more quickly than a secondmonomer component, the matrix material can include a higher percentageof the first component than the gel. Alternatively or in addition,multiple polymer components, e.g., with different hardnesses, can bepresent in the polishing layer 112 to achieve desired materialproperties and polishing characteristics. This combination of polymerswithin a layer can provided by a randomized blend or structured layout.

In addition, in any of the implementations of FIGS. 2A-2E, the backinglayer 114 can be composed of a solid polymer matrix material 204, e.g.,substantially without pores, e.g., porosity less than 1%, e.g., lessthan 0.5%. The matrix material of the backing layer 114 can be softerthan the matrix material polishing layer 112. The matrix material of thepolishing layer 112 and the matrix material of the backing layer 114 canbe formed from the same monomer or polymer components, but in differentweight percentage contributions. For example, assuming after curing thata first monomer component polymerizes more quickly or forms a harderpolymer matrix than a second monomer component, the polishing layer 112can include a higher percentage of the first component than the backinglayer 114.

FIG. 2A shows a polishing layer 112 having a solid matrix material 200and a plurality of pores 202. The region 119 above the acoustic window118 includes pores 202 but lacks grooves 116. For example, the pores inthe region 119 above the acoustic window 118 can have the same porosity,e.g., density and size of pores, as the remainder of the polishing layer112. Conventional polishing pads typically include pores in thepolishing layer, e.g., hollow polymer microspheres. Without beinglimited to any particular theory, in contrast to hollow pores whichsignificantly increase acoustic attenuation, the liquid-filled pores areacoustically transmissive and thus do not increase acoustic attenuationsuch that a special window region is needed for the polishing layer 112.

The portion of the backing layer 114 directly above the acoustic sensor162 can include an acoustic window 118. The acoustic window 118 has alower acoustic attenuation coefficient than the surrounding backinglayer 114. The acoustic impedance of a material is a measure of theopposition that a material presents to the acoustic flow resulting froman acoustic pressure applied to the material. The acoustic attenuationcoefficient quantifies how transmitted acoustic amplitude decreases as afunction of frequency for a specific material.

The material of the acoustic window 118 has a sufficiently low acousticattenuation coefficient, e.g., to provide a signal satisfactory foracoustic monitoring. The window can have an acoustic attenuationcoefficient lower than 2 (e.g., lower than 1, lower than 0.5). toprovide a signal satisfactory for acoustic monitoring. In general, theacoustic attenuation coefficient should be as low as possible (i.e., noabsorption). The acoustic impedance of the window 118 can be between 1and 4 MRayl. For example, the acoustic impedance of the window 118 canbe reasonably close to water, e.g., about 1.4 MRayl.

In addition or as an alternative to a low attenuation coefficient, inimplementations in which a window 118 sits between a portion 119 of thepolishing layer 112 and the sensor 162, e.g., as shown in FIGS. 2A and2B, the acoustic refractive index of the window 118 can be selected toprovide index matching. The acoustic refractive index sets the speed ofsound within a material. In particular, for the frequencies beingmeasured, acoustic refractive index of the window 118 can be between(inclusive of the endpoints) the acoustic refractive index of theportion 119 of the polishing layer 112 and the material that contactsthe bottom of the window, e.g., the top plate of the sensor 162 or anindex matching gel. In particular, the acoustic refractive index of thewindow 118 can be equal to the acoustic refractive index of the portion119 of the polishing layer 112. Alternatively, the acoustic refractiveindex of the window 118 can be between, but not equal to, the acousticrefractive index of the portion 119 of the polishing layer 112 and thematerial that contacts the bottom of the window. Index matching reducesreflections of acoustic energy at interfaces and thus can improvetransmission of acoustic energy to the sensor 162.

In the implementations of FIGS. 2A-2D, the acoustic window 118 is formedof a different material than the backing layer 114. This permits thebacking layer 114 to be composed of a wider range of materials to meetthe needs of the CMP operation. The acoustic window 118 can be composedof a non-porous material, e.g., a solid body. For example, the acousticmaterial can be a polymer, e.g., a polyurethane. In someimplementations, the acoustic window 118 and the backing layer 114 areformed using the same two monomer or polymer components, but indifferent weight percentage contributions. The percentage for thebacking layer 114 can be selected so as to provide the desiredcompressibility, whereas the percentage for the window 114 can beselected so as to provide the desired acoustic transmission. In someimplementations, the acoustic window 118 is a gel. In someimplementations, the acoustic window 118 is a gel material.

In the implementations of FIGS. 2A-2D, the acoustic window 118 can be asolid polymer matrix material with liquid-filled pores 203 (see FIG.2F). The polymer matrix material of the window 118 can be the samecomposition as the polymer matrix material of the polishing layer 112.The liquid in the pores 203 of the window 118 can be the samecomposition as the liquid in the pores of the polishing layer 112.However, to provide increased acoustic transmittance, the matrixmaterial of the window 118 can less compressible than the polymer matrixmaterial of the polishing layer 112. In addition, the pores 203 in thewindow can be smaller or dispersed at a lower density than the pores 202in the polishing layer 112. In addition, the liquid in the pores 203 inthe window can have a different viscosity, e.g., a higher viscosity,than the liquid in the pores 202 in the polishing layer 112.

Where pores 202/203 are present in the polishing layer 112 and/or window118, the pores can occupy from 1 to 40% by volume of the layer orwindow. The pores 202/203 can be 10 to 300 μm in width (parallel to thepolishing surface) and 2 to 40 μm in depth (perpendicular to thepolishing surface); the pores 202/203 can be wider than they are deep.The pores 203 in the window can be narrower and/or shorter and/or occupya lower percentage of volume than the pores 202 in the polishing layer112.

The acoustic window 118 can be formed integrally with the surroundingbacking layer 114 (and polishing layer 112 if appropriate). Inparticular, the material of the window 118 and the material of thebacking layer 114 can be intermingled at the interface. For example, ifthe window 118 and backing layer 114 are formed by ejection of dropletsof different liquid precursor materials, the droplets can interminglealong the boundary before being cured. Similarly, if the window 118extends through the polishing layer 112, then the window 118 andpolishing layer 112 can be formed by ejection of droplets of differentliquid precursor materials, and the droplets can intermingle along theboundary before being cured. As such, adhesive is not needed to securethe window 118 to the polishing pad 110.

The acoustic window 118 can be wider than the acoustic sensor 162, e.g.,as shown in FIG. 2A, or the two can be of substantially equal width(e.g., within 10%). Where the acoustic window 118 is narrower than theacoustic sensor 162, the sensor can also abut the bottom of the backinglayer 114.

The acoustic sensor 162 is a contact acoustic sensor 162 having asurface connected to (e.g., in direct contact with, or having just anadhesive layer) a portion of the backing layer 114 and/or the acousticwindow 118. For example, the acoustic sensor 162 can be anelectromagnetic acoustic transducer or piezoelectric acoustictransducer. A piezoelectric sensor can include a rigid contact plate,e.g., of stainless steel or the like, which is placed into contact withthe body to be monitored, and a piezoelectric assembly, e.g., apiezoelectric layer sandwiched between two electrodes, on the backsideof the contact plate.

The acoustic sensor 162 can be secured to a portion of the backing layer114 and/or to the acoustic window 118 by an adhesive layer. The adhesivelayer increases the contact area between the acoustic sensor 162 and thebacking layer 114 and/or acoustic window 118, reduces undesirable motionin the acoustic sensor 162 during polishing operations, and can reducethe presence of gas pockets between the acoustic sensor 162 and thebacking layer 114 and/or acoustic window 118 thereby improving thecoupling to the sensor, thus reducing noise in the acoustic signalreceived by the acoustic sensor 162. The adhesive layer 170 can be aglue applied between the acoustic sensor 162 and the backing layer 114and/or acoustic window 118, or an adhesive strip (e.g., tape). Forexample, the adhesive layer can be a cyanoacrylate, pressure sensitiveadhesive, hot melt adhesive, etc. However, in some implementations, theacoustic sensor 162 contacts the acoustic window 118 directly.

The acoustic window 118 extends through the backing layer 114 such thatone surface, e.g., an upper surface, contacts a lower surface 112 b ofthe polishing layer 112. The opposing surface, e.g., a bottom surface,can be coplanar with a lower surface of the backing layer 114.

The acoustic window 118 can be composed of a non-porous material. Ingeneral, non-porous materials transmit acoustic signals with reducednoise and dispersion compared to porous materials. The acoustic window118 material can have a compressibility within a range of thecompressibility of the surrounding matrix material 204 that reduces theeffect of the acoustic window 118 on the polishing characteristics ofthe polishing layer 112. In some implementations, the acoustic window118 compressibility is within 10% of the matrix material 204compressibility (e.g., within 8%, within 5%, within 3%). In someimplementations, the acoustic window 118 is opaque to light, e.g.,visible light. The acoustic window 118 can be composed of one or more ofpolyurethane, polyacrylate, polyethylene, or another polymer that has asufficiently low acoustic impedance coefficient.

The acoustic window 118 is shown extending through the total thicknessof the backing layer 114. However, the bottom of the acoustic window 118could be recessed relative to the bottom of the backing layer 114. Theacoustic sensor 162 extends through an aperture in the platen 120 tocontact the underside of the window 118.

In some implementations, the acoustic monitoring system 160 includes anacoustically transmissive layer, e.g., an index-matching material,between the acoustic sensor 162 and the window 118. Assuming an adhesiveis used, the acoustically transmissive layer can be in contact with theadhesive layer which provides increased acoustic signal coupling betweenthe elements in contact with the transmissive layer. The transmissivelayer can be arranged between the acoustic window 118 and the adhesivelayer, or between the adhesive layer and the acoustic sensor 162. Insome implementations, the acoustic monitoring system 160 includes theadhesive layer, the transmissive layer, or both. For example, thetransmissive layer can be a layer of Aqualink™, Rexolite, or Aqualene™.In some implementations, the transmissive layer has an acousticattenuation coefficient that is within 20%, e.g., 10%, of the acousticattenuation coefficient of the acoustic window 118. The acousticallytransmissive layer can have an acoustic attenuation coefficient lessthan the acoustic attenuation coefficient of the surrounding backinglayer 114.

FIG. 2B shows an implementation of the pad 110 having a region 119 inthe polishing layer 112 in which the region 119 does not includes pores202, but can otherwise be constructed as described for theimplementation of FIG. 2A. Without wishing to be bound by theory, theacoustic signal dispersion through the region 119 can be reduced bydecreasing the number of changes of acoustic refractive index associatedwith multiple material boundaries, such as for example, the contact areabetween the matrix material 200 and the pores 202. Therefore theacoustic signal which reaches the acoustic window 118 and acousticsensor 162 arranged beneath can have decreased dispersion and noise.Thus, in this implementation the window 118 might not include pores.

In some implementations, the acoustic window 118 extends through thethickness of the pad 110. As shown in FIG. 2C, the acoustic window 118extends through both the polishing layer 112 and the backing layer 114.Here, the acoustic window 118 has a lower acoustic impedance than thesurrounding polishing layer 112 and backing layer 114. The acousticwindow 118 is positioned such that the top surface of the acousticwindow 118 is coplanar with the polishing surface 112 a, and the bottomsurface of the acoustic window is coplanar with the lower surface of thebacking layer 114, e.g., lower surface 114 b, that contacts the platen120. The acoustic sensor 162 contacts the exposed surface of theacoustic window 118 and receives the transmitted acoustic signal.

The acoustic window 118 is formed of a different material than thepolishing layer 112. This permits the backing layer 114 to be composedof a wider range of materials to meet the needs of the CMP operation. Inother respects, the implementation of FIG. 2C be constructed asdescribed for the implementations of FIGS. 2A-2B.

FIG. 2D illustrates an implementation that is similar to FIG. 2A or 2B,but the window 118 in the backing layer 114 is provided by a portion ofthe polishing layer 112 that projects downward into an aperture 114 a inthe backing layer 114. Thus, the window 118 forms a unitary body withthe rest of the polishing layer 112, i.e., there is no gap,discontinuity of material composition, etc., that provides a seambetween the two portions. The bottom surface of the protruding portion118 of the polishing layer can be coplanar with the lower surface 114 bof the backing layer 114.

In addition, although FIG. 2D illustrates the region 119 of thepolishing layer 112 as lacking pores (such as in FIG. 2B), the window118 can include liquid-filled pores 202 (such as those shown in FIG.2A). Either the upper section that spans the thickness of the polishinglayer 112 can include liquid-filled pores 202, or the lower protrudingportion can include liquid-filled pores 202, or both.

Referring to FIG. 2E, in some implementations the acoustic transmissionof both the polishing layer 112 and the backing layer 114 aresufficiently high that an acoustic window is not required. In this case,the acoustic sensor 162 can be placed in direct contact with the lowersurface 114 b of the backing layer 114.

Conventional polishing pads 110 typically include a porous backing layer114. Without being limited to any particular theory, the pores 202increase the acoustic impedance of the backing layer 114. However, byforming the backing layer 114 of a non-porous but compressible material,a significantly lower acoustic impedance can be achieved, thus enablingmonitoring of an acoustic signal without requiring a window 118 for thebacking layer 114. In addition, as described above, the liquid-filledpores 202 are also acoustically transmissive and thus do not increaseacoustic impedance such that a window 118 is needed for the polishinglayer 112. Moreover, the window 118 in the backing layer 114 can haveliquid-filled pores. be

In some implementations, the acoustic monitoring system 160 includes anactive acoustic monitoring system. Such implementations include anacoustic signal generator and an acoustic sensor, such as acousticsensor 162.

The active acoustic generator generates (i.e., emits) acoustic signalsfrom a side of the substrate closer to the polishing pad 110. Thegenerator can be connected by circuitry 168 to a power supply and/orother signal processing electronics 166 through a rotary coupling, e.g.,a mercury slip ring. The signal processing electronics 166 can beconnected in turn to the controller 190, which can be additionallyconfigured to control the magnitude or frequency of the acoustic energytransmitted by the generator, e.g., by variably increasing or decreasingthe current supply to the generator. The acoustic signal generator 163and acoustic sensor 162 can be coupled to one another, though this isnot required. The sensor 162 and the generator can be decoupled andphysically separated from one another. For the generator, commerciallyavailable acoustic signal generators can be used. The generator can beattached to platen 120.

In some implementations a plurality of acoustic sensors 162 can beinstalled in the platen 120 beneath respective acoustic windows 118.Each acoustic sensor 162 has an associated acoustic window 118. Eachsensor 162 can be configured in the manner described for any of FIGS. 1and 2A-2E. In some implementations, such as the implementation of FIG. 3, the plurality of sensors 162 can be positioned at different angularpositions around the axis of rotation of the platen 120, but at the sameradial distance from the axis of rotation. In particular, the sensors162 can be distributed at equal angular intervals around the axis ofrotation. In some implementations, the plurality of sensors 162 arepositioned at different radial distances from the axis of rotation ofthe platen 120, but at the same angular position. In someimplementations, the plurality of sensors 162 are be positioned atdifferent angular positions around and different radial distances fromthe axis of rotation of the platen 120, as shown in the implementationof FIG. 3 .

In some implementations, the acoustic window 119 is surrounded by asmooth portion 174 of the polishing layer 112. As shown in FIG. 4 , thesmooth portion 174 lacks grooves 116 and is coplanar with the uppersurface of the acoustic window 119. Implementations including a smoothportion 174 surrounding the acoustic window 119 can reduce noiseassociated with the substrate 10 interacting with the grooves 116 of apolishing layer 112 during a polishing operation.

Turning now to the signal from the sensor 162 of any of the priorimplementations, the signal, e.g., after amplification, preliminaryfiltering and digitization, can be subject to data processing, e.g., inthe controller 190, for either endpoint detection or feedback orfeedforward control.

In some implementations, the controller 190 is configured to monitoracoustic loss. For example, the received signal strength is compared tothe emitted signal strength to generate a normalized signal, and thenormalized can be monitored over time to detect changes. Such changescan indicate a polishing endpoint, e.g., if the signal crosses athreshold value.

In some implementations, a frequency analysis of the signal isperformed. For example, frequency domain analysis can be used todetermine changes in the relative power of spectral frequencies, and todetermine when a film transition has occurred at a particular radius.Information about time of transition by radius can be used to triggerendpoint. As another example, a Fast Fourier Transform (FFT) can beperformed on the signal to generate a frequency spectrum. A particularfrequency band can be monitored, and if the intensity in the frequencyband crosses a threshold value, this can indicate exposure of anunderlying layer, which can be used to trigger endpoint. Alternatively,if a location (e.g., wavelength) or bandwidth of a local maxima orminima in a selected frequency range crosses a threshold value, this canindicate exposure of an underlying layer, which can be used to triggerendpoint. For example, for monitoring of polishing of inter-layerdielectric (ILD) in a shallow trench isolation (STI), a frequency rangeof 225 kHz to 350 kHz can be monitored.

As another example, a wavelet packet transform (WPT) can be performed onthe signal to decompose the signal into a low-frequency component and ahigh frequency component. The decomposition can be iterated if necessaryto break the signal into smaller components. The intensity of one of thefrequency components can be monitored, and if the intensity in thecomponent crosses a threshold value, this can indicate exposure of anunderlying layer, which can be used to trigger endpoint.

Assuming the positions of the sensors 162 relative to the substrate 10are known, e.g., using a motor encoder signal or an optical interrupterattached to the platen 120, the positions of the acoustic events on thesubstrate can be calculated, e.g., the radial distance of the event fromthe center of the substrate can be calculated. Determination of theposition of a sensor relative to the substrate is discussed in U.S. Pat.No. 6,159,073 and in U.S. Pat. No. 6,296,548, incorporated by reference.

Various process-meaningful acoustic events include micro-scratches, filmtransition break through, and film clearing. Various methods can be usedto analyze the acoustic emission signal from the waveguide. Fouriertransformation and other frequency analysis methods can be used todetermine the peak frequencies occurring during polishing.Experimentally determined thresholds and monitoring within definedfrequency ranges are used to identify expected and unexpected changesduring polishing. Examples of expected changes include the suddenappearance of a peak frequency during transitions in film hardness.Examples of unexpected changes include problems with the consumable set(such as pad glazing or other process-drift-inducing machine healthproblems).

In operation, as a device substrate 10 is being polished at thepolishing station 100, an acoustic signal is collected from the in-situacoustic monitoring system 160. The signal is monitored to detectexposure of the underlying layer of the substrate 10. For example, aspecific frequency range can be monitored, and the intensity can bemonitored and compared to an experimentally determined threshold value.

Detection of the polishing endpoint triggers halting of the polishing,although polishing can continue for a predetermined amount of time afterendpoint trigger. Alternatively or in addition, the data collectedand/or the endpoint detection time can be fed forward to controlprocessing of the substrate in a subsequent processing operation, e.g.,polishing at a subsequent station, or can be fed back to controlprocessing of a subsequent substrate at the same polishing station. Forexample, detection of the polishing endpoint can trigger modification tothe current pressures of the polishing head. As another example,detection of the polishing endpoint can trigger modification to thebaseline pressures of the subsequent polishing of a new substrate.

Implementations and all of the functional operations described in thisspecification can be implemented in digital electronic circuitry, or incomputer software, firmware, or hardware, including the structural meansdisclosed in this specification and structural equivalents thereof, orin combinations of them. Implementations described herein can beimplemented as one or more non-transitory computer program products,i.e., one or more computer programs tangibly embodied in a machinereadable storage device, for execution by, or to control the operationof, data processing apparatus, e.g., a programmable processor, acomputer, or multiple processors or computers.

A computer program (also known as a program, software, softwareapplication, or code) can be written in any form of programminglanguage, including compiled or interpreted languages, and it can bedeployed in any form, including as a standalone program or as a module,component, subroutine, or other unit suitable for use in a computingenvironment. A computer program does not necessarily correspond to afile. A program can be stored in a portion of a file that holds otherprograms or data, in a single file dedicated to the program in question,or in multiple coordinated files (e.g., files that store one or moremodules, sub programs, or portions of code). A computer program can bedeployed to be executed on one computer or on multiple computers at onesite or distributed across multiple sites and interconnected by acommunication network.

The processes and logic flows described in this specification can beperformed by one or more programmable processors executing one or morecomputer programs to perform functions by operating on input data andgenerating output. The processes and logic flows can also be performedby, and apparatus can also be implemented as, special purpose logiccircuitry, e.g., an FPGA (field programmable gate array) or an ASIC(application specific integrated circuit).

The term “data processing apparatus” encompasses all apparatus, devices,and machines for processing data, including by way of example aprogrammable processor, a computer, or multiple processors or computers.The apparatus can include, in addition to hardware, code that creates anexecution environment for the computer program in question, e.g., codethat constitutes processor firmware, a protocol stack, a databasemanagement system, an operating system, or a combination of one or moreof them. Processors suitable for the execution of a computer programinclude, by way of example, both general and special purposemicroprocessors, and any one or more processors of any kind of digitalcomputer.

Computer readable media suitable for storing computer programinstructions and data include all forms of non-volatile memory, mediaand memory devices, including by way of example semiconductor memorydevices, e.g., EPROM, EEPROM, and flash memory devices; magnetic disks,e.g., internal hard disks or removable disks; magneto optical disks; andCD ROM and DVD-ROM disks. The processor and the memory can besupplemented by, or incorporated in, special purpose logic circuitry.

The above described polishing apparatus and methods can be applied in avariety of polishing systems. Either the polishing pad, or the carrierhead, or both can move to provide relative motion between the polishingsurface and the wafer. For example, the platen may orbit rather thanrotate. The polishing pad can be a circular (or some other shape) padsecured to the platen. Some aspects of the endpoint detection system maybe applicable to linear polishing systems (e.g., where the polishing padis a continuous or a reel-to-reel belt that moves linearly). Thepolishing layer can be a standard (for example, polyurethane with orwithout fillers) polishing material, a soft material, or afixed-abrasive material. Terms of relative positioning are used; itshould be understood that the polishing surface and wafer can be held ina vertical orientation or some other orientations.

While this specification contains many specifics, these should not beconstrued as limitations on the scope of what may be claimed, but ratheras descriptions of features that may be specific to particularembodiments of particular inventions. In some implementations, themethod could be applied to other combinations of overlying andunderlying materials, and to signals from other sorts of in-situmonitoring systems, e.g., optical monitoring or eddy current monitoringsystems.

What is claimed is:
 1. A chemical mechanical polishing apparatus,comprising: a platen; a polishing pad supported on the platen, thepolishing pad including a polishing layer comprising a solid matrixmaterial with liquid-filled pores, and a backing layer; a carrier headto hold a surface of a substrate against the polishing layer of thepolishing pad; a motor to generate relative motion between the platenand the carrier head so as to polish an overlying layer of thesubstrate; an in-situ acoustic monitoring system comprising an acousticsensor coupled to the backing layer to receive acoustic signals from thesubstrate; and a controller configured to detect a polishing transitionpoint based on received acoustic signals from the in-situ acousticmonitoring system.
 2. The apparatus of claim 1, wherein the backinglayer comprises a first matrix material including a first polymer and asecond polymer and the polishing layer comprises a second matrixmaterial having pores, the second matrix material including the firstpolymer and the second polymer in a different percentage contributionthan the first matrix material that the backing layer is morecompressible than the polishing layer.
 3. The apparatus of claim 1,wherein the backing layer is non-porous.
 4. The apparatus of claim 1,wherein the polishing layer has a polishing surface with a first regionwithout grooves and a second region that surrounds the first region thathas a plurality of grooves, and wherein the acoustic sensor is alignedwith the first region.
 5. The apparatus of claim 1, wherein thepolishing layer comprises a porous portion and a non-porous solidportion that extends through the thickness of the polishing layer, andthe acoustic window is arranged within the backing layer in contact withthe solid portion.
 6. The apparatus of claim 1, wherein the controlleris configured to indicate an exposure of an underlying layer in responseto detection of the polishing transition point.
 7. The apparatus ofclaim 1, wherein the controller is configured to change a polishingparameter in response to detection of the polishing transition point. 8.The apparatus of claim 7, wherein the polishing parameter comprises acarrier head pressure or a polishing liquid composition.
 9. Theapparatus of claim 8, wherein the controller is configured to cause adispenser to switch from dispensing a first polishing liquid to a secondpolishing liquid having a lower polishing rate or lower selectivity inresponse to detection of the polishing transition point.
 10. Theapparatus of claim 1, wherein the controller is configured to haltpolishing in response to detection of the polishing transition point.11. A method of fabricating a polishing pad, comprising: successivelydepositing a first plurality of layers with a 3D printer to form abacking layer of the polishing pad, each layer of the first plurality oflayers including a backing material portion and an acoustic windowportion having a lower acoustic impedance than the backing materialportion, the backing material portion and the acoustic window portiondeposited by ejecting a plurality of precursor materials from one ormore nozzles and solidifying the plurality of precursor materials toform a solidified backing material and a solidified acoustic window,wherein ejecting the plurality of precursor materials from the one ormore nozzles forms an interface with intermingled polymer that directlybonds the solidified acoustic window and the solidified backingmaterial; and successively depositing a second plurality of layers withthe 3D printer to form a polishing layer of the polishing pad on thebacking layer.
 12. The method of claim 11, wherein the ejecting theplurality of precursor materials comprises ejecting a first precursormaterial for the backing material portion and a ejecting a secondprecursor material for the window portion.
 13. The method of claim 11,wherein ejecting the plurality of precursor materials includes ejectinga first precursor and a second precursor for the backing materialportion and ejecting the first precursor and the second precursor in adifferent percentage contribution than the backing material portion toform the window portion.
 14. The method of claim 11, wherein ejectingthe plurality of precursor materials includes ejecting a first precursorand a second precursor for the backing material portion, and whereinsuccessively depositing a second plurality of layers includes ejectingthe first precursor and the second precursor in a different percentagecontribution than the backing material portion to form the polishinglayer.
 15. The method of claim 11, wherein successively depositing thesecond plurality of layers with the 3D printer to form the polishinglayer includes ejecting a liquid material that forms liquid filled poresat selected voxels in the polishing layer after curing of the secondplurality of layers.
 16. The method of claim 15, comprising ejecting theliquid material at a region in the polishing layer over the window inthe backing layer.
 17. The method of claim 15, comprising refrainingfrom ejecting the liquid material at a region in the polishing layer thewindow in the backing layer such that the portion is non-porous.
 18. Themethod of claim 11, wherein each layer of the second plurality of layersincludes a polishing material portion and a second acoustic windowportion having a lower acoustic impedance than the polishing materialportion, the polishing material portion and the second acoustic windowportion deposited by ejecting a second plurality of precursor materialsfrom the one or more nozzles and solidifying the second plurality ofprecursor materials to form a solidified polishing material and asolidified acoustic window material, wherein ejecting the secondplurality of precursor materials from the one or more nozzles forms ansecond interface with intermingled polymer that directly bonds thesolidified acoustic window and the solidified polishing material.
 19. Amethod for manufacturing a polishing pad, comprising: depositing abacking layer comprising a first material; curing the backing layer;depositing a polishing layer atop the backing layer, the polishing layercomprising a second material; curing the polishing layer; removing aportion of the backing layer creating an aperture; and inserting anacoustic window comprising a third material into the aperture.
 20. Themethod of claim 19, further comprising, between removing the portion ofthe backing layer and inserting the acoustic window, removing a portionof the polishing layer corresponding with the portion of the backinglayer.
 21. The method of claim 19, wherein the second material comprisesa polymer matrix with liquid-filled pores.
 22. The method of claim 19,wherein the third material has a same compressibility as the firstmaterial.